Heat oven to 350°F; line sheets with parchment or silicone mats.

Beat butter and both sugars in large bowl 2-3 minutes until creamy and fluffy.

Add eggs and vanilla, beating thoroughly after each addition.

Separately whisk flour, baking soda, and salt; gradually mix into wet just until combined.

Gently fold in 1 cup M&Ms and all chocolate chips; reserve remaining 1/2 cup M&Ms.

Drop rounded tablespoons 2 inches apart; press 2-3 reserved M&Ms on each.

Bake 9-11 minutes until edges lightly golden and centers soft. Do not overbake; cookies keep cooking on pan.

Cool on sheet 5 minutes; transfer to rack to cool completely.